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4.1 Technical Principles and Status Quo

Energetic use

In today's energy system, the energetic use of hydrogen plays a subordinate role. In the course of increased sector coupling, however, especially green hydrogen could also be increasingly used as an greenhousegas-neutral energy source in the future, for example in the mobility sector. The energetic use of hydrogen can basically take place in two different processes. In addition to classical combustion, there is the possibility of using hydrogen in fuel cells. In the following, the basic principles of these two technologies for the energetic use of hydrogen are described.

Fuel Cells

In a fuel cell, the chemical energy bound in the hydrogen is converted directly into electrical energy. Due to the direct conversion without a thermal process - as it takes place in a classic gas turbine, for example - high efficiencies can be achieved with the help of fuel cell technology. Depending on the design, fuel cells can be operated not only with hydrogen but also with a hydrogen-containing fuel gas, such as natural gas (CH4). The design of a fuel cell is similar to that of an electrolytic cell and is shown schematically in the illustration below.

Schematic design of a fuel cell
Schematic design of a fuel cell by Fabian Carels, Klell et al. (2017) (CC BY-SA)


The core elements of every fuel cell are two electrodes which are separated from each other by a gas-impermeable electrolyte. The hydrogen supplied to the fuel cell is directed to the anode. There, two H+ ions are formed from one hydrogen molecule (H2) by a so-called oxidation reaction. In addition, electrons (e-) are released:

                                                                       H2 --> (2 H+) + (2 e-)

These electrons are taken up by the anode and travel via an electrical conductor to the cathode. The flow of electrons can be used as an electric current via an external circuit.
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Oxygen is supplied to the cathode at the same time.  In many fuel cell types, ambient air can be supplied instead of pure oxygen. The electrons from the anode cause the oxygen molecules to be split into O2 ions. In a final step, the H+ ions react with the O2 ions to water. The exchange of ions takes place via the ion-conducting electrolyte. Whether the reaction of the ions with each other takes place at the anode or the cathode depends on the design of the fuel cell.
 
Water is the only waste product that is produced when hydrogen is used in fuel cells. The overall reaction of all processes taking place in the fuel cell can be described with the following equation:

                                                        2 H2 + O2 --> 2 H2O + Electricity + Heat

In practice, fuel cells achieve electrical efficiencies of up to 60 % and are thus significantly more efficient than conventional thermal combustion processes. In addition to electrical energy, the waste heat generated during the reactions can also be used. Similar to electrolysis cells, in practice a large number of individual fuel cells are interconnected to form so-called stacks in order to generate higher voltages. 
 
There are many different types of fuel cells, which differ from each other, for example, in terms of the materials used, the operating temperature or the efficiency. At present, the most common type of fuel cell is the Proton Exchange Membrane Fuel Cell (PEMFC). The PEMFC is particularly well suited for applications in mobility, such as hydrogen-powered trucks, due to its flexible mode of operation and compact design. For stationary applications, i.e. the reconversion of hydrogen into electricity, the solid oxide fuel cell (SOFC) can be a suitable alternative. The SOFC is characterised by particularly high electrical efficiencies and is operated at temperatures above 800 degrees celcius, which simplifies the utilisation of waste heat.

Combustion
  
An energetic use of hydrogen is not only possible in fuel cells but also within a conventional combustion process. During combustion, the energy bound in the hydrogen is converted into thermal energy. The generated thermal energy can then be used directly, for example to supply industrial processes or to heat water in order to supply residential buildings. It is also possible to use the thermal energy released in the combustion process to produce electrical energy. 
 
The conversion of thermal energy into electricity is the basis of conventional, fossil-based electricity generation in coal and gas-fired power plants. After a technical adaptation, technologies known from the conversion of natural gas into electricity, such as gas turbines or gas engines, may also be used for the generation of electricity from hydrogen. Regardless of whether natural gas or hydrogen is burnt; part of the thermal energy released cannot be converted into electricity and is instead released in the form of waste heat. In the so-called combined heat and power (CHP) process, this waste heat is made technically available and used, for example, to heat buildings.

Hydrogen combustion is with regard to the underlying reaction principles comparable to the combustion of natural gas. However, hydrogen combustion has some special characteristics:

  • No CO2 is produced during the combustion of hydrogen. The only reaction product of the combustion is water vapour.
  • The temperature that results from the combustion of hydrogen is higher than that of natural gas combustion.
  • Due to the higher combustion temperatures, the formation of nitrogen oxide (NOx) is enhanced and, without countermeasures, is higher than with the combustion of natural gas. Nitrogen oxides are pollutants and have a negative effect on people's lung function.
  • When hydrogen is burnt, the spread of the flame into areas not intended for this purpose can lead to problems. The reason for this spread is the very high flame velocity of hydrogen (several times higher compared to natural gas), which results in a more difficult to control combustion of the gas. The flame velocity describes the speed at which unburnt gas spreads in the flame.
  • Compared to natural gas, hydrogen combustion is connected to some additional safety risks. For example, less energy is required for the ignition of hydrogen, which means that spontaneous combustion can occur more easily if hydrogen accidentally emerges (e.g. in the case of damaged pipelines). In addition, the flame that is formed during the combustion of hydrogen is almost invisible. This leads, for example, to the fact that fire brigades depend on infrared cameras to identify and extinguish the source of the fire in accidents where hydrogen is burning.

The challenges for safe hydrogen combustion can be solved by technological adaptations. Thus, the development and optimisation of combustion systems for hydrogen is currently being worked on. Some manufacturers already offer smaller gas turbines and gas engines that are suitable for operation with hydrogen.